Chalk is a variety of limestone. It is a rock composed of the mineral calcite (calcium carbonate). It is unique amongst limestones in that it can form at any latitude and in all but the deepest oceans. Its production is governed by single-celled marine algae; floating phytoplankton which secrete calcium carbonate from their cells. These sub-microscopic algae are called coccolithophores and billions can ‘bloom’ in nutrient-rich seas. On death the inorganic calcite sinks to the sea bed to form an extremely pure, biogenic limestone. The British Chalks was deposited from the Upper Cretaceous to Early Palaeogene, a time when the world witnessed very high sea-levels and low continental relief. Although coccoliths are tiny, Chalk deposition continued for almost 35 million years, accumulating layers at least 1-2 kms thick in northern Europe. The rock is white, untainted by impurities and relatively soft, enabling chalk pigments, or ‘whitings’, to be produced by simply grinding the stone to a fine powder.
 Mortimore, R., 2011, A chalk revolution: what have we done to the Chalk of England?, Proceedings of the Geologists’ Association 122, 232–297.